“A lot depends on how the state of Missouri would implement this rule,” Driscoll said. “There's a lot of flexibility, and a lot of options.”

Driscoll said if Missouri opts to reduce its reliance on coal by shifting to low-carbon sources of energy, or by using its energy resources more efficiently, the state could see substantial improvements in air quality.

On the other hand, Missouri might be able to find ways to continue burning coal ― if, for example, the state were able to buy "carbon credits" from other states that brought their carbon emissions down below federally-mandated targets.

This map shows regional changes in summertime ozone concentrations under "scenario 2" of the Syracuse/Harvard analysis. That scenario assumed a 36% reduction in carbon dioxide emissions by 2020, compared to 2005 levels.